170 BELL SYSTEM TECHNICAL JOURNAL 



mile. This was one of the cases where the vessel was well south 

 and close in shore and, of course, represents very poor transmission. 

 It means that with the vessel traveling as slowly as ten miles (16 km.) 

 per hour, a transmission change of about five units per hour will 

 occur. In telephone practice it is desirable to keep the transmission 

 equivalent constant to within two or three units, so that this condi- 

 tion would require re-adjusting the amplification as often as every 

 half hour. 



These curves show the necessity for so designing the receiving set 

 as to be able readily to obtain a wide variation in amplification in 

 order to accommodate the changes in transmission efficiency. Under 

 the conditions of the test, it was found that a variation of 40 units 

 amplification is necessary in order to carry the vessel from a range of 

 40 up to 200 statute miles (64 to 320 km.). In order to gain such 

 control, it is desirable to switch stages of amplification into and out 

 of the circuits and to obtain closer adjustment by means of networks 

 of variable loss. 



When we speak of holding the circuit to a constant transmission 

 equivalent thruout a wide variation in the position of the ship, we 

 do not mean that the circuit for 200 miles (320 km.), for example, 

 is as good as that for 40 miles (64 km.). The field strength received 

 over the longer circuit is, of course, very much weaker than that 

 received over the shorter one, and is subject to correspondingly more 

 interference. For a transmitting power of about one kilowatt used 

 in the tests, it was found that the circuit was rather consistently 

 good up to 100 miles (160 km.) or so. During summer daylight 

 condition, it was only fairly good at around 150 miles (240 km.), and 

 at 200 miles (320 km.) was subject to so much interference that its 

 insurance against interruption in service was small. Under more 

 favorable conditions, particularly as at night, in the winter time, 

 connections could be established over very much greater distances, 

 but not reliably. 



Field Strength Measurements 



The circuit transmission measurements described above were sup- 

 plemented during the latter end of the experiments by measurements 

 of a more fundamental nature, namely, of the received field strength. 

 These measurements represent the application to the ship-to-shore 

 development program of what was really another investigation — that 

 of the development of methods and apparatus for measuring field 

 strengths at these relatively short wave lengths as well as for longer 

 wave lengths. The method and means employed will not be de- 



